The invention relates to a signalling method in a wireless digital telecommunication system and to a wireless digital telecommunication system.
In digital radio systems, call-related signalling between a base station and a terminal occurs simultaneously with a call to be transferred on a traffic channel. The GSM system, for example, employs two signalling channels for conducting call-related signalling, a SACCH (Slow Associated Control Channel) and a FACCH (Fast Associated Control Channel). The SACCH is a separate low-rate signalling channel associated with each traffic channel. Due to its low rate the channel can be used for non-urgent signalling only. The FACCH is fast signalling within the traffic channel, and can be used for time-critical signalling. However, a part of the traffic channel capacity is then lost.
In digital radio systems, such as the GSM system, data transmission occurs in bursts, and a specific burst structure is determined for each purpose, like data and signalling transmission, synchronization or equalization. A normal burst used for data and signalling transmission comprises in the middle a training sequence including a set of predetermined symbols that the receiver identifies. When comparing the received training sequence with a known training sequence the receiver may create information on the distortion that an un-ideal radio path causes to the received signal. On the basis of this information the receiver can demodulate the received signal more efficiently. The applicant""s previous patent application PCT/FI97/00465 describes a signalling method, in which a training sequence is used for signalling, that speeds up the signalling and leaves the entire traffic channel to be used by the payload.
The problem with the above system is the number of different training sequences required for transmitting versatile signals. One training sequence can describe one signalling message only, and if, for example, power control signalling including eight steps is to be performed, eight different training sequences are required. If the aim is to perform several signalling events with the above method, the number of training sequences easily increases significantly. This increases the receiver load and complicates the implementation of the receiver, since the received training sequence must be separately compared with each known training sequence. Particularly distinguishing training sequences from one another becomes more difficult when the reception conditions are poor. Furthermore, when higher data transmission rates are aimed at, the use of a training sequence no longer necessarily guarantees fast enough signalling owing to the above and since the duration of the training sequence is relatively long bearing in mind that the training sequence is capable of transmitting one message only. Due to these drawbacks said method has not been widely employed and the GSM system, for example, still uses signalling channels.
A significant drawback with prior art signalling is that it matches poorly with packet-switched data transmission. In present packet-switched wireless telecommunication systems, such as the GPRS system (General Packet Radio Service) designed on the basis of the GSM system, data transmission from a terminal to a base station (uplink) and vice versa (downlink) takes place irrespective of one another. In said systems a channel is formed between the terminal and the base station only, when either party has data packets to send. In other words, there is no need to maintain a continuous return channel for transmitting signalling or acknowledgement messages in said systems. To utilize such asymmetrical data transmission optimally would require transmission of signalling and acknowledgement messages when the channel capacity is used for other purposes as well.
The object of the invention is thus to provide an improved method and an apparatus implementing the method to avoid the above problems. The objects of the invention are achieved by a method and a system, which are characterized in what is said in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on the idea that various training sequences are used in the burst structure to indicate the contents of the burst. The recipient then interprets the received burst on the basis of the training sequence. A particular training sequence can preferably be employed in the burst structure to indicate that signalling data in particular is transmitted using the burst. The type of the signalling data to be transmitted can also be indicated in more detail by means of the stealing symbols in the burst. Stealing symbols have typically been used in a normal burst to indicate whether the data bits in a burst are allocated to be used on a traffic channel or stolen to be used on the FACCH. However, in the method of the invention the stealing symbols in the normal burst to be utilized for transmitting user data can also be allocated to transmit user data.
The method and system of the invention provide an advantage that enables transmission of signalling and acknowledgement messages particularly when the channel between the terminal and the base station is also employed for transferring user data. This is particularly advantageous in packet-switched wireless data transmission, as in this case the channel between the terminal and the base station does not have to be separately opened for signalling. Another advantage of the invention is that the data transmission capacity of a normal burst to be used for transmitting user data is not used for the signalling of the invention. A further advantage of the invention is that it alleviates the implementation of the detection in the receiver, as the received training sequence is compared with the two known training sequences only, thus reducing the receiver load. Still another advantage of the invention is that the data transmission capacity of a normal burst increases, since the stealing symbols in the normal burst can also be utilized for transmitting user data.